On Sun, 2 Dec 2007 08:03:56 -0800 (PST),
wrote:


Also, there's nothing magic about the MOSFET, all you really need is
a logic level N channel device capable of handling the solenoid
current when it's turned on and holding off the solenoid voltage
when it's turned off.

Hi John, I would like to get back to the MOSFET comment. This sounds
scary to me. About the current. Do you mean, as we both calucated that
if the 72V will use 3A current, the then mosfet may need 5V 3A, in and
that will allow the 72V 3A out?

---
No. What I meant was that when there's zero volts on the gate, the
MOSFET will be turned OFF and the drain will be required to stand
off the supply voltage. On the other hand, when the gate is at 5V
the MOSFET will be turned ON and the solenoid current will flow
through the MOSFET from drain to source.

There is no current required into or out of the gate except what's
needed to charge and discharge the gate capacitance.
---

So my pic needs to give 3A??? This sounds almost impossible. I'll have
to check the datasheets. Is this how all transistors work re current:
The current in the base needs to be the same as the current you want
in/out on the collector and emitter?

---
No. for bipolar transistors there's a characteristic named 'beta'
(or Hfe) which specifies the current gain of the transistor:

Ic
Hfe = ----
Ib

For example, if a transistor is specified as having a minimum beta
of 100 with 100mA of collector current, then the maximum base
current required to get that 100mA of collector current will be:

Ic 100mA
Ib = ----- = ------- = 1mA
Hfe 100

For MOSFETs, in this type of application, all that's necessary is to
raise the voltage on the gate high enough to lower the resistance of
the drain-to-source channel to the point where its resistance is
negligible with respect to the load.
---

And then of course, my next question will be if you can suggest an
appropriate mosfet from my local shop: www.radionics.ie

---
Which circuit are you planning on using?


--
JF